1. Field of the Invention
The present invention relates to a two- or more-channel digitally controlled DC/DC converter using fixed ON-time nonlinear control.
2. Description of the Related Art
Methods for controlling a DC/DC converter using analog control include nonlinear control called hysteresis control. In this hysteresis control, an output voltage feedback system does not include an error amplifier, and phase compensation is unnecessary. Accordingly, a very-high-speed response can be provided.
FIG. 4 is a diagram showing one example of a two-channel power supply circuit using hysteresis control. FIG. 5 is a diagram showing an input ripple current waveform of two-channel hysteresis control.
A switching power supply circuit of a first channel includes switching elements 21 and 31, a driver 81 for alternately driving the switching elements 21 and 31, and an LC filter 91. The driver 81 is controlled by a negative feedback circuit based on a comparator CMP1. Similarly, a switching power supply circuit of a second channel includes switching elements 22 and 32, a driver 82 for alternately driving the switching elements 22 and 32, and an LC filter 92. The driver 82 is controlled by a negative feedback circuit based on a comparator CMP2. It should be noted that in FIG. 4, Vref refers to a reference voltage. In this example of a power supply circuit shown in FIG. 4, the current waveform of the first channel and the current waveform of the second channel overlap each other as shown in FIG. 5.
Moreover, in a DC/DC converter using PWM control, a ripple current through an input capacitor can be reduced by shifting the phases of switching of respective channels of the two- or more-channel DC/DC converter to reduce overlaps between ON times of switching elements of the respective channels.
FIG. 6 is a diagram showing a two-channel power supply circuit using PWM control. FIG. 7 is a diagram showing an input ripple current waveform of the two-channel power supply using PWM control.
A switching power supply circuit of a first channel includes switching elements 21 and 31, a driver 81 for alternately driving the switching elements 21 and 31, and an LC filter 91. The driver 81 is controlled by a negative feedback circuit based on an error amplifier AMP1. Similarly, a switching power supply circuit of a second channel includes switching elements 22 and 32, a driver 82 for alternately driving the switching elements 22 and 32, and an LC filter 92. The driver 82 is controlled by a negative feedback circuit based on an error amplifier AMP2. It should be noted that in FIG. 6, Vref refers to a reference voltage.
It should be noted that the following fact has also been known: in the case where a nonlinearly controlled DC/DC converter is stably controlled in a certain steady operating state, a frequency f is constant and determined by an input voltage Vin, an output voltage Vo, and an ON time ton of a switching element 2i. In the case of a non-isolated synchronous rectification step-down DC/DC converter such as shown in FIGS. 1 and 4, the frequency f is represented by the following expression (1):f=Vo/(Vin×ton)   (1)
Japanese Patent Application Laid-Open Nos. 2010-259280, 2011-259686, 2013-062966, 2013-243875, and the like disclose switching power supplies, but none of them are intended to reduce a ripple current through an input capacitor.
To realize a high-speed response in a DC/DC converter, nonlinear control such as hysteresis control is needed because linear control such as PWM control has limitations. It should be noted, however, that if nonlinear control such as hysteresis control is applied to a two- or more-channel DC/DC converter, overlaps between ON times of the switching elements increase depending on the phases of switching of respective channels. This increases a ripple current through an input capacitor, and affects the life of the input capacitor. Meanwhile, if linear control such as PWM control is applied to a two- or more-channel DC/DC converter, a ripple current through an input capacitor can be reduced because it is easy to shift the phases of switching of the respective channels, but it is difficult to provide a high-speed response.